J. Min. Metall. Sect. B-Metall. 54 (3) B (2018) 313 - 322 Journal of Mining and Metallurgy, Section B: Metallurgy

CARBOTHERMIC REDUCTION OF MECHANICALLY ACTIVATED NiO- MIXTURE: NON-ISOTHERMAL KINETICS

S. Bakhshandeh, N. Setoudeh *, M. Ali Askari Zamani, A. Mohassel

Yasouj University, Materials Engineering Department, Yasouj, Iran

(Received 23 March 2018; accepted 10 December 2018) Abstract

The effect of mechanical activation on the carbothermic reduction of nickel was investigated. Mixtures of nickel oxide and activated carbon (99% carbon) were milled for different periods of time in a planetary ball mill. The unmilled mixture and milled samples were subjected to thermogravimetric analysis (TGA) under an argon atmosphere and their solid products of the reduction reaction were studied using XRD experiments. TGA showed that the reduction of NiO started at ~800 and ~720 in un-milled and one-hour milled samples respectively whilst after 25h of milling it decreased to about 430 . The kinetics parameters of carbothermic reduction were determined using non-isothermal method (Coats-Redfern Method) for un-milled and milled samples. The activation energy was determined to be about 222 kJ mol -1 for un-milled mixture whilst it was decreased to about 148 kJ mol -1 in 25-h milled sample. The decrease in the particle size/crystallite size of the℃ milled samp℃les resulted in a significant drop in the reaction temperature. ℃ Keywords : Ball milling; Carbothermic reduction; Nickel oxide; Non-isothermal kinetics; Thermo-gravimetric analysis (TGA).

1. Introduction solid product catalyzes the reaction rate [ 11 ]. It has been shown that mechanical activation by There have been many investigations on the milling reactants together with extended periods of carbothermic reduction of metal oxide in the literature time can intensify the reactions [ 16-19 ]. During and the mechanism of this reaction has been the mechanical activation and ball milling processes, the subject of intense research [ 1-5 ]. Recently, the use of reactant materials are intimately mixed at the solar energy for production of metals from their nanoscale. Previous works have indicated the rate of with carbothermal process has been a subject carbothermic reduction reactions can be greatly of the study to optimize the carbothermal reduction of increased by pre-milling the mineral and carbon MgO using concentrated solar energy in low vacuum together compared to with powders milled separately conditions [ 6]. Several mechanisms have been and then mixed [ 16-19 ]. The effect of mechanical proposed to explain the carbothermic reduction activation and extended milling on the carbothermic between two solid reactants (such as oxide and reduction of manganese ore [ 18 ], ilmenite [ 20 ], carbon) and the formation of solid metal. The oldest hematite [ 21 ], oxides [ 22 ] celestite [ 19 , 23 ], and most widespread mechanism in this regard is the and carbonitridation of zircon [24] have been studied oxide reduction through gaseous intermediate CO (g) recently. The results show that, generally, the longer and CO 2 in accordance with the following reactions the samples are milled together the greater the extent [7]: of reaction at lower temperatures is [ 24 ]. Reduction of MO+Æ+ CO M CO nickel oxide by graphite during ball milling at both ()sg ( ) () s2 (1) ambient and elevated temperatures has been studied C + (s CO2 +Æ C()sg2 CO ( ) by Yang and McCormick [ 25 ]. Their results showed (2) that milling significantly reduced the critical reaction N + (g The carbothermic reduction of nickel oxide and temperature for reduction [ 25 ]. reduction of NiO with CO (g) have been studied under There are many research works about the different conditions by many researchers [ 8-15 ]. It has carbothermic reduction of nickel oxide. However, the been reported that the reduction of NiO is attributed to kinetics of reduction reaction between NiO and the special class of solid-state reactions where the carbon has been studied under an isothermal condition a *Correspon d i n g a u t h o r : n s e t o u d e h @ y u . a c . i r a a DOI:10.2 2 9 8 / J M M B 1 8 0 3 2 3 0 2 2 B a a a [ [ [ [ [ [ a a [ [ 1 1 - - ( ( 1 1 1 1 3 1 3 1

314 S. Bakhshandeh et al. / JMM 54 (3) B (2018) 313 - 322 in the previous works [ 8, 11-14 ]. Reduction of nickel thermogravimetric analyzer (BAHR thermoanalyzer- oxide with carbon, polyethylene, and polystyrene has STA503-GmbH). Physical mixtures of raw materials been investigated under non-isothermal conditions in (NiO-Carbon active) was prepared in accordance with the temperature range 20-1000 . The authors the stoichiometric mixture given by reaction (3) in indicated that at heating rates of 5-20 K/min, both order to study the carbothermic reduction in the un- carbon and polyethylene reduce nickel oxide. milled samples, Moreover, they determined that the effective All solid products were analyzed using X-ray activation energies of NiO reductio℃n with carbon are diffraction (XRD, Philips Analytical, Co-K α about 284 kJ mol -1 (68 kcal mol -1 ) [ 15 ]. radiation, 40 kV, 40 mA, X’Pert APD) over a 2 θ range The present work was conducted in order to study of 20-100° and 2 sec count time for every 0.05° step. the effects of mechanical activation on the The microstructures of the as-milled and the heated carbothermic reduction of nickel oxide with activated samples were studied using a scanning electron carbon. The aim of this work is to investigate the microscope (TESCAN-VEGA3). The crystallite size kinetics of carbothermic reduction of nickel oxide of solid products in the as-milled samples was under non-isothermal conditions for un-milled determined using the well-known Scherrer equation. mixture and milled samples. The kinetics parameters for both un-milled and milled samples are determined 3. Results and discussion using Coats-Redfern method. Moreover, the 3.1 Characterization of mixtures fundamental characteristics of both un-milled mixture and milled samples are examined using TGA, XRD, Fig. 1 illustrates the XRD patterns of the as-milled and SEM analyses. samples for different milling times. The major peaks of NiO (JCPDS card file No. 71-1179) were clearly 2. Experimental observed in the 25h as-milled sample. Therefore, based on the XRD results in Fig. 1, reduction of nickel The characteristics of starting materials (nickel oxide by carbon active cannot occur after 25h milling. oxide and carbon active with purity of 99%) are listed Thermodynamic calculations [ 26-27 ] indicate that in Table 1. The amount of ash for carbon active was reaction (3) is endothermic and feasible at less than 1%. Stoichiometric mixtures of NiO-Carbon temperatures above of 440 . Therefore, the signs of (86.148 wt.% of NiO and 13.852 wt.% of carbon NiO in the XRD patterns of as-milled samples (Fig.1) active) were prepared based on reaction (3). clearly show that the temperature of solid mixtures in NiO+Æ C Ni + CO the milling vial can never reach 440 ()g (3) The broadening ℃of the main peak of NiO at È g Tl hermodynamic calculations indicate the value of 2 50.7° (d=2.08794Å, 200) with increasing milling ΔG0 for reaction (3) is negative at a temperature above time (Fig. 1) may be related to the reduction of mean ~440 at standard conditions [ 26,27 ]. crystallite size. It is worth noting ℃th aatf treerd 2uc5t ihoonu rosf mofe manil clirnygs.t allite size of particles and amorphization, a Table 1. The characteristics of starting materials. well-known phenomenon during high-energy ball a milling, results in not only decreased peak intensity Characteristic Nickel oxide (NiO) Carbon active but broadening of the peak width as well [ 28-29 ]. of m℃ater i a l s a Table 2 lists the mean crystallite sizes of the nickel Colou r b l a c k b l a cak oxide for different milling times. The crystallite size Structure Cubic (JCPDS-71-1179) amorphous [ of nickel phase was determined using Scherrer d(0.1) = 0.86 d(0.1) = 4.961 equation based on its main peak (i.e., 2=50.705). The Size analyses d(0. 5 ) = 6 . 1 5 3 d ( 0 . 5 ) = [ 18.806 results in Table 2 show that with increasing milling (m) d(0.9 ) = 2 4 . 8 0 5 d ( 0 . 9 ) = [ 58.392 time, the crystallite size of nickel oxide decreases and reaches about 20 nm in 25h milled sample. The st o i c h i o m e t r i c m i x t u r e s w e r e m e c h aanically milled in a closed chamber using a planetary ball mill Table 2. The values of crystallite size of nickel oxide and [ percent of contamination in the as-milled (Farapazhouhesh, FP2 model) for different milling samples with increasing milling times. times. The milling c o n d i t i o n s f o r a l l r u n s 1were: a rotation speed of 600 rpm, high-chromium steel balls Milling time Crystallite size Amount of Iron - (20 mm diameter), high-chromium steel milling (hr.) (nm) (in ppm) ( chamber, and the ball-to-powder weight ratio of 40:1. 1 38.76 6.61 Thermograv i m e t r i c a n a l y s i s ( T G A ) w a s c a r r i1ed 1out 5 28.91 12.41 on the NiO-carbon mixtures at the heating rate of 10 10 25.56 11.54 C/m3in up t o 1 , 2 0 0 u n d e r t h e f l o w o f 1argon 15 25.78 13.92 (99%purity, flow rate of 5 mL/min) using a 25 19.84 14.72

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Figure 1. XRD patterns of the as-milled samples in different milling times.

A weak sign is observed at about 52° in the 25h Fig. 3 shows the TGA traces of un-milled and milled milled sample. Although elemental nickel has a peak stoichiometric mixtures of NiO-Carbon under pure at 2  52°, however thermodynamic calculations [26- argon atmosphere. The mass loss up to ~430 in the 27] indicate that carbothermic reduction of nickel 25h milled sample is likely due to desorption of gas oxide cannot occur at a temperature below 440 . The from activated carbon. An increase in gas sorption onto trace of a new peak at 2  52° disappeared after acid activated carbon after milling alone and with harder washing of the 25h as-milled sample with dilute phases has been demonstrated previously [24℃, 30-32]. hydrochloric acid (1 N). The acid washing was done The main mass loss starts in un-milled and one-hour in order to remove any iron contamination ℃in the milled samples at 800 and 720 , respectively; milled powders due to abrasion milling media and ball however, it begins in 25h milled sample at a lower collisions events during milling operations. The temperature (~430 ). According to reaction (3), the percentage of the iron content in the milled powders is complete reduction of nickel oxide with carbon should listed in Table 2. Clearly, the content of iron cause a weight loss of 32℃.3%. This eve℃nt is observed in contamination increases with milling time. Therefore, TGA curve for un-milled and 1h milled mixtures at the trace of a new peak at about 52° may be attributed ~1000 and at ~63℃0 for 25h milled sample. The total to iron contamination during milling operations. mass losses of samples in the TGA analyses (Fig. 3) are Fig. 2 presents SEM micrographs of raw mixtures consistent with the theoretical value of 32.3% calculated and milled powders. It is clear from Fig.2a that the from reaction (3). Therefore, TGA results indicate that particle sizes of carbon content in the raw mixture are the re℃action betwe℃en NiO and carbon active is much larger than those of agglomerated nickel oxide completed in both un-milled mixture and 1h milled particles. A decrease in the size of particles with samples after heating in argon atmosphere up to milling time was observed in the SEM micrographs of ~1000 . The results of TGA graphs also show that milled mixtures (Figs. 2b to 2d). The XRD results milling significantly reduces the critical reaction (Fig. 1) show that reduction reaction of nickel oxide to temperatures from 800 in the un-milled sample to metallic nickel cannot occur during mechanical ~430 for the 25h milled sample. Table (2) indicates activation of NiO-Carbon mixture. However, milling that wi℃th increasing milling time, the crystallite size of significantly reduces the particle/crystallite size and NiO decreases to ~20 nm in the 25 -h milled sample. microstructural refinement is observed in the milled According to Fig.2, ℃the particle refinement also samples (Fig. 2). obser℃ves in the SEM micrographs of milled samples. Considering the XRD patterns in Fig. 1 and the Therefore, the microstructural refinement due to milling values in Table 2, 1h and 25h milled samples were operations results in significant reduction in the critical prepared for TGA under argon atmosphere. To temperature of TGA analyses. The previous study on the compare the results of milled samples with the un- reduction of NiO with graphite revealed that the milled mixture, TGA was also done on the un-milled decreasing in particle size during milling causes a stoichiometric mixture of NiO-Carbon under an argon significant reduction in reaction temperature during atmosphere flow. subsequent heating [ 25 ]. 316 S. Bakhshandeh et al. / JMM 54 (3) B (2018) 313 - 322 a) Carbon b)

c) d)

Figure 2. SEM micrographs of raw mixture and as-milled powders: (a) raw mixture; (b) 1 hour milled sample; (c) 15 hours milled sample; (d) 25 hours milled sample. Fig.3 shows a mass gain takes place in the 25h 25h milled samples have three stages of mass loss. milled samples after heating above ~630 and Those traces of mass losses are same for both samples continues up to 1100 . It seems that a reaction to being up to ~500 . Heating beyond 500 results in occurred in the temperature range of 630-1100 under continuing mass loss in the 1:1.5 ratios (NiO:Carbon argon atmosphere for 25h milled sample. It may be molar ratio of 1:1.5) and reaching about 38% at presumed that the reduced metallic nickel pha℃se (the 800 . Traces of nickel oxide are not observed for solid product of reac℃tion 3) reacts with an impurity of 1:1.5 sample ℃in the XRD pattern of the s℃olid residuals argon atmosphere (oxygen). This scenario conf℃irms the of the TGA analysis. Although few mass gains are presence of traces of nickel oxide peak in the XRD observed beyond heating 800 in the 1:1.5 molar analysis of the solid residual sample of TGA analysis ratio℃s, however the results of Fig.4 show that the under argon atmosphere in 25h milled sample. addition of carbon more than stoichiometric ratio is Other mixtures of NiO-Carbon were prepared to effective and plays a significant role in the give NiO:Carbon molar ratio of 1:1.5 (50% excess of carbothermic reaction of nickel℃ oxide. stoichiometry according to reaction 3). These new Fig. 5 presents the rates of mass losses (dTGA) for mixtures were also milled in a planetary ball mill for un-milled mixture and milled samples. Those traces 25h under milling conditions same as stoichiometric were driven from Fig. 1 by differentiating the curves NiO-Carbon mixtures. Fig 4 indicates the TGA graphs mathematically. The curves demonstrate the reaction for 25h milled samples with stoichiometric mixture regions in the un-milled mixture and milled samples. and NiO-Carbon mixture with a molar ratio of 1:1.5 The dTGA graphs (Fig. 5) indicate that there is only under argon atmosphere. It is clear from Fig.4 that the one single continuously smooth peak evident in all S. Bakhshandeh et al. / JMM 54 (3) B (2018) 313 - 322 317

Figure 3. TGA traces of un-milled mixture and milled sample of NiO-Carbon with stoichiometric ratio under pure argon atmosphere.

Figure 4. TGA graphs for 25 hours milled mixture of NiO-Carbon with stiochiometric ratio and 25 hours milled mixture of NiO-Carbon with molar ratio of 1:1.5 under argon atmosphere. samples (un-milled and milled samples), confirming single peaks in dTGA graphs decrease with increasing the presence of a single reaction. In the case of milling time. The peak temperature is about 870 in multiple sequential reactions in the reduction of nickel the un-milled mixture and, finally, is reduces to oxide by carbon, the peak is expected to be ~470 in the 25h milled sample. Therefore, Fig. 5 discontinuous with shoulders or other peaks becoming shows that an increase in milling time is accompanied evident. Fig. 5 show that the temperatures of those by a decrease in the critical reduction temperatur℃e.

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Figure 5. The rates of mass losses (dTGA) for un-milled mixture and milled samples driven from figure 3. The peak temperature (T p) of each sample has been indicated at dTGA graph. a a a 3.2 Non-isothermal kinetics study where w 0 and w t are masses of the sample initially a n d a t t h e t i m e ( t ) r e s p e c t i v e l y a n d B a is the fractio n o f a The kinetics of carbothermic reduction of nickel mass loss for complete reduction reaction of NiO to oxide in the un-milled and 1h and 25h milled samples n i c k e l . T h e s e d a t a c a n b e o b t a i n e d f arom TGA graphs [ were studied by TGA under non-isothermal (Fig.3) for e a c h s a m p l e . F i g . 6[ illustrates the [ conditions. The kinetics parameters can be obtained variations of the extent of reaction ( ) versus from non-isothermal rate laws by both model-fitting temperature u s i n g e q u a t i o n ( 5 ) f o[ r the un-milled [ and isoconversional (model-free) methods [ 3, 33-34 ]. mixture and 1 h a n d 2 5 h m i l l e d s a m p[les. Fig. 6 sho w s a Model-fitting methods involve fitting different that the required temperatures for starting the a models to extent of reaction ( α) versus temperature carbothermic re a c t i o n o f N i O d e c r e a s e w i t h a n a a [ curves ( α- temperature) and simultaneously increase in milling time. The reaction temperatures in [ 1 determining the activation energy (E) and frequency Fig. 6 are in re a s o n a b l e a g r e e m e n t w i t h r e s u l t s o f a a - factor (A) [ 3]. There are several non-isothermal Table 2 a n d F i g s . 2 a n d 3 . T h e r e1 f o r e , t h e r e s u l t s a a model-fitting methods, with the Coats-Redfern demonstrate the significant role of a decrease in - ( method being among the most popular ones [3 , 34 ]. crystallite/particle sizes of NiO c a u s e d b y m i l l i n g o n [ [ ( 1 1 By plotting the left-hand side of equation (4), which the carbothermic reaction kine t i c s . [ [ includes +the integral reaction model versus B y i n s e r t i n g v a r i o u s i n t e g r a l r e a c t1ion 1 mo3dels, g( α) 1 [ (temperature in Kelvin), the activation energy (E a) and in Table 3 into equation (4), Ar r h e n i u s p a r a m e t e r s a r e [ 3 1 È g()a ˘ frequency factor (A) can be obtained from the slope ln 2 a determined for al l s a m p l e s f r o m p l o t t i n g Í T ˙ a and intercept of the curve, respectively [ 3]. 1 Î ˚

È g()a ˘ È AR Ê 2RT ˆ˘ E versus T . The temperature ranges f o r a l l s a m p l e s a r e ( [ [ ln =-ln 1 - a Í 2 ˙ Í Á ˜˙ (4) selected using TGA and dTGA graphs (Figs. 3 and 5). Î T ˚ Îb EaaË E ¯˚ RT 1 1 The integral reaction models in Table 3 are inserted in w where R, β, and T̅ in equation (4) are gas constant equation (4) and t h e m o d e l t h a t g i v e s t h e b e s t l i n e a r f i t - - -1 -1 -1 (8.314 J.mol K ), heating rate (k min ), and mean is selected as the o p t i m u m o n e . M e a n w h i l e , i t i s ( ( experime n t a l t e m p e r a t u r e , r e s p e c t i v e l y . S aeveral proved that the theoretical values of pre-exponential reaction models including differential reaction model, factors for solid-state r e a c t i o n s a r e i n t h e r a g e o f 1 0 6 - 1 11 1 a 18 -1 f( α), and integral reaction model are listed in Table 3. 10 s [3]. 3 3 1 1 The extent of reAaction ( α) for all samples can be Using variations of extents of reaction (Fig. 6) and a È g()a ˘ 1 calculated using the thermogravimetric data (Fig. 3) ln a by plotting Í 2 ˙ versus curves for all models by the following equati o n [ 3 ] : [ Î T ˚ T ()ww0 - t in Table 3, the first order model (Mampel) resulted in a = [ (5) ( ()Bw0 the best linear fit. a [ a a a [ [ 1 [ - ( [ a 1 1 3 1 [ 1 - ( 1 1 3 1

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Table 3. The common reaction rate models in the solid-state reactions [3, 34]. Reaction model f( ) g( ) Nucleation Models

3 1 Power law 4a 4 a 4 2 1 Power law 3a 3 a 3 1 1 Power law 2a 2 a 2 3 1 Avrami-Erofeev 41( --- aa )[ ln( 1 - )] 4 [ln( aa--1 a )]4 2 1 3 a 3 Avrami-Erofeev 3(11-a)[-- ln(a )] [- ln(1-a )] 1 [ 1 Avrami-Erofeev 21()[ln()]--aa 1 - 2 [[ln(- 1-a )]2 Diffusion Models [ [a 1 a -1 [ a 2 One dimensional diffusion 2 [ 2 1 a 1 2 Diffusion control (Jander) 21( -aa)[3 1-- ( 1 )] 3 - 1 1[(11--a )]3 - 1 2 3 [(11 -- a ) 3 ] - 1 [1----2 aa()1 3 Diffusion control (Crank) 2 3 1 ( Reaction order and geometrical contr-acati o n m o d e l s - Mampel (first order) 1-a -- (1ln(1 1 a) 2 -1 Second order ()31-a () 1111--a 1 1 1 21()-a 2 1--() 1 a 2 Contracting cylinder 3 1 2 1 31()-a 3 11--()a 3 Contracting sphere È 1 l

Figure 6. The variations of extend of reaction ( ) versus temperature using equation (5) for un-milled mixture, 1 hour and 25 hours milled samples.

Fig. 7 presents the corresponding plots of the controlled mechanism for carbothermic reduction of first-order model for the un-milled mixture and 1h nickel oxide. The set of Arrhenius parameters for and 25h milled samples. The first order model the un-milled mixture and 1h and 25h milled (Mampel) in Table 3 shows the chemically- samples is presented in Table 4. 320 S. Bakhshandeh et al. / JMM 54 (3) B (2018) 313 - 322 a) b)

a a a a a a a a [ [ a c) [ [ [ [ [ [ a a [

[ [ a 1 1 [ - - 1 ( ( - 1 1 1 1 ( 3 3 1 1 1 1 È g()a ˘ 1 ln 3 1 ( a Figure 7. The plots of ÎÍ T 2 ˚˙ versus T for first-order model (Mampel) of:

a) un-milled mixture; b) 1( hour and c) 25 hours milled samples.

Table 4. Arrhenium parameters for non-isothermal NiO+Æ+ CO()gg Ni CO2 (g ) (6) carbothermic reduction of nickel oxide according to first order (Mampel) model. If the nickel oxide particles cannot be in contact with carbon particles, it is expected that the major Sample un-milled 1 hr milled 25 hrs. milled reducing role in the carbothermic reaction of nickel Activation energy E oxide is that of CO (g) , which is formed according to 222 215 148 (kJ mol -1 ) Boudouard reaction (reaction 2). It has been reported Pre-exponential that the reaction between NiO and CO (g) is fast 2.34 10 9 4.09 10 9 6.28 10 9 constant A compared to Boudouard reaction [ 11 ]. In other words, the rate of carbothermic reduction of NiO is 3.3 Reaction kinetics parameters controlled by the rate of CO (g) supplying using Boudouard reaction. The autocatalytic nature of the The previous investigations about the reduced nickel metal phase is also known and the carbothermic reduction of metal oxide indicate that a reduced nickel metal phases catalyze the Boudouard direct reaction between the solid oxide and solid reaction in progressing reduction reaction [ 11 ]. Nickel carbon is important only when the gaseous products is also known to be one of the active catalysts for gasification of activated and less valuable of the reaction, CO and CO 2, are immediately released from the reaction system [ 1]. The direct reaction can coals at low temperature [ 35 ]. Therefore, it is only proceed at the contact points between the two concluded that Boudouard reaction plays a significant role in the carbothermic reduction of nickel oxide. solid reactants (metal oxide and carbon) [ 1]. 0 Therefore, the carbothermic reduction of NiO takes The standard free energy ( ΔG ) of Boudouard place in accordance with reaction (6) and Boudouard reaction is negative at above 700 at the standard reaction (reaction 2). atmospheric pressure. Thermodynamic assessments

℃ S. Bakhshandeh et al. / JMM 54 (3) B (2018) 313 - 322 321 also show that the lowest temperature for of NiO depends upon the rate at which carbon atoms carbothermic reduction of NiO by carbon (reaction 3) reach Ni/NiO interface as well as the extent of is about ~440 under the standard atmospheric reduction of NiO [ 14 ]. The rate at which carbon pressure [ 26-27 ]. The onset temperature of atoms reach NiO/Ni interface should depend upon carbothermic for un-milled mixture and 1hour milled the concentration of active sites in the carbon sample are ~800 and ~770 , respectively (Figs. 3 surface and contact area between the active sites in and 5). Figs. 3 a℃nd 5, however, indicate that the onset the carbon surface and the Ni surface [ 14 ]. It is clear temperature of 25h milled sample reduces to ~ 400 . that mechanical milling increases the interfacial Many researchers have reported different values area and reduces the diffusion distances of reactants of activation ene℃rgy for car℃bothermic reduction of powders. Decreasing in particle/crystallite size of NiO under isothermal conditions [ 8, 10 , 11 , 14 , 15 ]. reactants (Table 2 and Fig. 2) during milling Sharma et al. [ 12-14 ] studied the reduction of N℃iO operations causes a significant reduction in the with natural graphite in vacuum and reported an -1 -1 reaction temperature of carbothermic reduction of activation energy of 313 kJmol (75 kcal mol ) using NiO. Hence, a decrease in onset carbothermic the first-order plots. The activation energy value for reduction temperature can be attributed to the reduction of NiO by active charcoal powder was -1 -1 microstructural refinement observed in the 25h reported to be about 207kJmol (49.47 kcal mol ) for milled sample. the non-catalytic carbothermic case [ 11 ]. Krasuk et al. [8] observed the first-order reduction reaction of NiO 4. Conclusions by CO (g) and reported an activation energy of 196 kJmol -1 (47 kcal mol -1 ) over the temperature range of The results of TGA graphs and the XRD 566 to 682 . The activation energy of the Boudouard -1 analyses of the samples showed that the reaction h a s b e e n r e p o r t e d t o b e 2 0 0 - 2 5 0 k J m o al [1]. The present work showed that the activation energy carbothermic reduction of nickel oxide could be values fo r u n - m i l l e d m i x t u r e a n d 1 h m i l l e d saample improved by extended milling. The mass losses are initiated at a lower temperature in 1h and 25h milled (Table 4) a ℃ r e i n g o o d a g r e e m e n t w i t h t h o s e r e aported by Krasuk [ 8] and Satish [ 11 ]. The results in Table 4 samples than in the un-milled sample. The kinetics also show that the valu e s o f a c t i v a t i o n e n e r g y [for un- parameters of carbothermic reduction of nickel oxide were determined using non-isothermal milled mixture and 1h m i l l e d s a m p l e s a r e i[n good agreement with the activation energy of Boudouard method (Coats-Redfern method) for un-milled reaction. Therefore, the r e s u l t s o f p r e s e n t w o r k [ reveal mixture and milled samples. The kinetics results of that the ov e r a l l c a r b o t h e r m i c r e d u c t i o n o f N i O a in the the un-milled and milled samples revealed that the un-milled mixture and one-hour milled sample can rate of carbothermic reduction of NiO could be take place by solid-gas rea c t i o n s ( r e a c t i o n s 6 [ and 2) described by the first-order model (Mampel model). which are carried out at the same time. The kinetics results revealed that Boudouard 1 Table 4 shows that the activation energy for reaction (reaction 2) is the predominant reaction in carbothe r m i c r e d u c t i o n o f t h e 2 5 h m i l l e d s a m- ple is the carbothermic reduction of NiO. Therefore, the -1 about 148 k J m o l . T h i s v a l u e i s m u c h l o w e r t h(an the overall reduction reaction of nickel oxide occurred activation energy of Boudouard reaction. Therefore, by reactions (6) and (2) for un-milled and 1h milled the kinetics p a r a m e t e r s o f T a b l e 4 s h o w 1 t 1hat samples. The activation energy values of the Bou3douard r e a c t i o n c a n n o t b e t h e r a t e - c o n t r1olling carbothermic reduction of nickel oxide were 222 reaction in the 25h milled sample. It appears, kJmol -1 for un-milled mixture and 148 kJmol -1 for the therefore, tha t t h e reaction between NiO and carbon in 25h milled sample, respectively. Therefore, the 25h milled sample occurs through the following mechanical activation intensified carbothermic reaction: ) reduction of NiO by decreasing particle/crystallite 2NiO+Æ C2 Ni + CO() g 2 (7) sizes of reactants and by inducing increase thorough mixing of the reactants (NiO-Carbon). Thermodynamic assessments show the value of 0 ΔG for reaction (7) is negative at a temperature Acknowledgements above of 180 [26-27 ]. Meanwhile, according to reaction (7), the complete reduction of NiO with This work has formed a part of research master carbon must cause a mass loss of 27%, which is plan of the corresponding author (Nader Setoudeh) close to the amount of mass loss of 25h milled and has been supported by Yasouj University. The sample at 500 ℃. Fig. 3 shows that the percentage of authors gratefully acknowledge Deputy of Research the mass loss of 25h milled sample reaches about and Technology of Yasouj University for financial 30% at a temperature of 500 . support of the research project. Previous results show that the rate of reduction ℃

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KARBOTERMIČKA REDUKCIJA MEHANIČKI AKTIVIRANE SMEŠE NiO - UGLJENIK: KINETIKA U NEIZOTERMALNIM USLOVIMA

S. Bakhshandeh, N. Setoudeh *, M. Ali Askari Zamani, A. Mohassel

Univerzitet u Jasudžu, Odsek za nauku o materijalima, Jasudž, Iran Abstract

U ovom radu je ispitivan uticaj mehaničke aktivacije na karbotermičku redukciju nikl oksida. Smeša nikl oksida i aktivnog uglja (99% ugljenik) je usitnjena u planetarnom kugličnom mlinu tokom različitih vremenskih perioda. Neusitnjena smeša i usitnjeni uzorci su podvrgnuti termogravimetrijskoj analizi (TGA) u argonskoj atmosferi, a zatim su čvrsti produkti nastali redukcijom ispitani putem rentgenske difrakcije (XRD). Termogravimetrijska analiza je pokazala da je redukija NiO počela na ~800℃ kod neusitnjenih uzoraka i na ~720℃ kod uzoraka koji su usitnjavani 1h, dok se kod uzoraka koji su usitnjavani 25h temperatura smanjila na 430℃. Kinetički parametri karbotermičke redukcije su određeni putem neizotermalne metode (Koats–Redfernova metoda) za neusitnjene i usitnjene uzorke. Aktivaciona energija za neusitnjenu smešu je iznosila oko 222 kJ mol -1 , dok je za uzorak koji je usitnjavan 25h iznosila oko 148 kJ mol -1 . Smanjenje veličine čestice/ kristalita kod usitnjenih uzoraka uzrokovala je značajno smanjenje reakcijske temperature.

Ključne reči: Sitnjenje u kugličnom mlinu; Karbotermička redukcija; Nikl oksid; Kinetika u neizotermalnim uslovima; Termogravimetrijska analiza (TGA).